Abstract

Carbon–iron oxide microspheres’ black pigments (CIOMBs) had been prepared via ultrasonic
spray pyrolysis of aqueous solutions containing ferrous chloride and glucose. Due
to the presence of carbon, CIOMBs not only exhibited remarkably acid resistance, but
also could be well dispersed in both polar solvents and nonpolar solvent. Finally,
dispersions of hollow CIOMBs in tetrachloroethylene had successfully been applied
in electrophoretic displays.

Keywords:

E-paper; Pigment; Black; Carbon

Introduction

Electrophoretic displays (EPDs) have been the subject of intense research and development
for a number of years [1-5]. In such displays, pigments are dispersed as a colloidal suspension in a colored
solvent, which is placed in a narrow space between two transparent electrodes. By
applying an electric field to the suspension, the charged pigments can be moved through
the fluid. When the field is applied in one direction, the pigments move toward the
observer and the pixel has the color of the pigments. When the field is applied in
the opposite direction, the ITO electrode closest to the observer is covered with
colored solvent. Just like printed paper, EPDs are lightweight, flexible and easy
to read in all lighting conditions [6-9]. Above of all, they provide an important improvement in the field of power consumption:
due to the bistability of the pigments, an image is retained on the display without
the need for constant refreshment. Their properties make them the most likely candidate
in the search for a display that combines the benefits of printed paper and an image
display. EPDs are now used in consumer products such as the Amazon Kindle or Sony
E-Reader. Nevertheless, problems with the long-term image quality of the displays
have prevented their widespread usage. For example, pigments that make up EPDs tend
to settle, resulting in inadequate service life for the displays [10-13].

The change in the appearance of a pixel of the EPDs is solely due to the change of
the position of the pigments within the suspension. Thus, the pigments play a vital
role on the speed and efficiency of the EPDs. For a white/black display cell, the
white particles are typically titanium oxide because of its relatively high refractive
index of 2.7, which leads to excellent scattering properties [14]. Generally, carbon black is the material of choice to produce the black color in
most applications. However, carbon black has a complex and poorly understood surface
chemistry, which may vary widely with the specific raw material and the exact process
used for the carbon black production. Furthermore, carbon black has a poorly understood
aggregate, fractal structure. It has also been discovered that carbon black has a
low density (0.8 g/cm3), which is not matched to that of the usual suspending medium (about 1.7 g/cm3) [15]. There is thus a need for black particle with proper density and surface chemistry
for used in EPDs that can be compatible with the suspending fluid.

Herein, we demonstrate that carbon–iron oxide microspheres’ black pigments (CIOMBs)
are formed via ultrasonic spray pyrolysis of the aqueous solutions containing FeCl2 and glucose. Different from the formation process of carbon powders, in situ-formed
iron oxide nanoparticles are introduced as surface chemistry and density modification
component to form carbon–iron oxide composites. The density of the CIOMBs can be easily
tuned in the range of 1.5–2.2 g/cm3 by acid etching, which resolved the problem of mismatched density between carbon-based
black pigment and electrophoretic medium. The CIOMBs can be well dispersed in various
solvents including polar solvents (water, methanol, ethanol, etc.) and nonpolar solvent
(e.g. tetrachloroethylene) due to their surfaces are functionalized with abundant
functional groups such as OH, COH and CH2. The CIOMBs can be used as effective black electrophoretic particle for electrophoretic
displays.

Experimental

Ferrous chloride (FeCl2), glucose, and HCl solution (12 M), titania, DISERBYK-161 were purchased and used
without further purification. Deionized water was used throughout this work.

The USP experimental setup is similar to that of Suslick’s group [16-19]. In a typical experiment, 0.5 mol/L glucose and 0.8 mol/L FeCl2 were added to deionized water so as to form a precursor solution under magnetic stirring.
For nebulization, a Yuyue household ultrasonic humidifier was used to nebulize a precursor
solution into a fine mist. The furnace temperature was set as 700°C with a nitrogen
flow rate of 2 SLPM (Standard Liters per Minute). After 1 h of collection into water-filled
beaker, the black product was collected with a magnet. After washed with deionized
water and then ethanol three more times, the black sample was collected for further
analysis.

In order to tune the density of CIOMBs, as-prepared CIOMBs were immersed in concentrated
HCl (12 M) at room temperature for 1–12 h. The content of iron oxide in the composite
dissolve in the concentrated HCl gradually, which leads to decrease in the CIOMBs’
density. Furthermore, CIOMBs were washed with deionized water and then ethanol three
more times, and the black sample was collected for further analysis.

Electrophoretic dispersions were prepared via dispersing of CIOMBs (0.05 g/mL) and
titania (0.10 g/mL) in tetrachloroethylene using DISERBYK-161 (0.01 g/mL) as a dispersant.
It is then introduced in an electrophoretic cell, composed of two ITO-coated glasses
(3.0 × 7.0 cm) sealed with 75 μm thickness, and the electrode area was prepared by
a photoetching technique. The cell is operated using a Keithley 230 programmable power
source.

X-ray diffraction (XRD) patterns were obtained on Rigaku Dmax2000 with Cu Kα radiation.
The morphology of the samples was examined by scanning (SEM, XL30ESEM FEG) and transmission
(TEM, JEOL 2010) electron microscopy. Fourier transform infrared spectrum (FT-IR)
of the samples was recorded at the ambient temperature on a Nigolet 560 Fourier Transform
Infrared Spectrometer with the resolution factor of ±0.1 cm−1.

Results and Discussions

In the present route, ultrasound is used to produce aerosol droplets dispersed in
nitrogen via ultrasonic spray treatment of precursor aqueous solution. The aerosol
droplets are carried in nitrogen stream through a quartz-glass tube in a hot furnace,
and water evaporation and precursor decomposition occur, generating CIOMBs. With USP,
the aerosol droplets will act as individual micrometer-sized chemical reactors to
impose morphology control on the target products.

Herein, ferrous chloride and glucose are used as precursor of iron oxide and carbon,
respectively, to form CIOMBs via USP. The morphology of the product is studied by
both scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
The overall morphology of CIOMBs is revealed in Fig. 1, which indicates that the sample is composed of a large quantity of core–shell microspheres
with the size in the range of 1–3 μm. From Fig. 1, we can see that the initial concentration of FeCl2 has influence on both the size and the morphology of CIOMBs. As shown in Fig. 1a, the size of CIOMBs is in the range of 300–1,400 nm when the concentration of FeCl2 is 0.08 M. The surface of as-prepared spheres is smooth. When the concentration of
FeCl2 is 0.32 M, the size of sample will increase to 500–3,000 nm. In addition, the surface
becomes rough, which comprises iron oxide nanoparticles with size of about 50–100
nm (the inset of Fig. 1b). Furthermore, microspheres with Fe-rich core/C-rich shell (the size is in the range
500–3,000 nm) are formed when the concentration of FeCl2 reaches up to 0.8 M. As shown in Fig. 1c, d, as-prepared spheres display two distinct surfaces: bubble-like surface and rough
surface. That is, some spheres have bubble-like carbon surface (consists of many bubble
in the carbon surface), and Fe-rich core comprises the agglomeration of iron oxide
nanoparticles. The other spheres have rough carbon-rich shell (the thickness is about
100 nm) and Fe-rich core (Fig. 1d and the inset of Fig. 1c).

The possible formation mechanism of CIOMBs can be illustrated as follows: (1) Carbonaceous
nanoparticles loaded with Fe2+/Fe3+ ion are generated via the catalyzing of Fe2+[20,21]; (2) With the furnace temperature increasing, carbon-coated iron oxide nanoparticles
will be formed via the oxidization of iron ion, accompanying by the further carbonization
of carbonaceous components with the catalysis of in situ-formed iron oxide [21]; (3) With the reaction time prolonging, more carbon will be formed in the neighborhood
of aforementioned iron oxide nanoparticles via the carbonization of glucose. When
the initial concentration of FeCl2 is low, as-produced CIOMBs display smooth surface, in which iron oxide nanoparticles
are randomly dispersed in the carbon spheres (Fig. 1a). When the initial concentration of FeCl2 is high enough, as-produced CIOMBs display rough surface due to aforementioned iron
oxide nanoparticles tend to agglomerate to form iron oxide-rich core (Fig. 1b–1d). With the reaction time prolonging, subsequently generated carbon will be formed
carbon shell around the iron oxide-rich core.

Figure 2 shows the XRD pattern of aforementioned CIOMBs. As shown in Fig. 2, the diffraction peaks can be indexed to the mixture of face-centered cubic Fe3O4 (JCPDS file No. 48-1487) and hexagonal-structured α-Fe2O3 (JCPDS file No.33-0664). There are no obvious crystalline peaks of carbon, indicating
that the structure of carbon is amorphous.

The FT-IR spectrum is used to identify the surface chemistry present in the CIOMBs.
Overall, in the range of 1,000–3,500 cm−1, the spectrum is similar to that of carbon spheres obtained via the hydrothermal
treatment of glucose aqueous solution [22]. As shown in Fig. 3, the bands at 3,400 and 1,625 cm−1 can be assigned to the O–H and C=C, respectively. The bands in the range 1,000–1,400
cm−1 can be attributed to C–OH stretching vibration and O–H bending vibrations. It should
be noted that the bands at 2,925 and 2,855 cm−1, which can be assigned to hydrogen bonded sp3 carbon [23], are stronger than those of the pure carbon sphere, while the band at 1,710 (C=O)
is almost disappear. The reason may lie on the relative higher reaction temperature
(700 vs. 180°C), which leads to the more complete carbonization of glucose. In the
case of three bands (455, 535 and 570 cm−1) in the range of 500–700 cm−1, they can be assigned to Fe3O4 and Fe2O3, respectively. Both Fe3O4 and Fe2O3 exist in the spectrum of sample CIOMBs, which are consistent with the results of
XRD (Fig. 3).

As demonstrated in the previous literatures, both crude carbon blacks and crude carbon
nanotubes are hard dispersed in either polar or nonpolar solvents. In the case of
carbon spheres obtained via the dehydration of polysaccharides aqueous solution under
hydrothermal conditions (<200°C), they exhibit excellent dispersibility in polar solvents
including water and ethanol. However, they are also hard to disperse in nonpolar solvent
such as tetrachloroethylene. The dispersibility of CIOMBs is investigated via dispersing
CIOMBs in various solvents. Poor dispersion usually limits their application in EPDs.
It is well known that surface’s functional groups play important roles in the process
of dispersion. As shown in Fig. 3, there are abundant functional groups (OH, COH, CH2, etc.) on the surface of CIOMBs, which lead to good dispersibility in various solvents.
The CIOMBs not only can be dispersed in polar solvents (i.e. water, methanol and ethanol),
but also can be dispersed in nonpolar solvent, such as tetrachloroethylene. The proved
surface chemistry makes CIOMB a versatile black pigment for EPDs.

The density of the as-prepared CIOMBs is about 2.2 g/cm3, which makes CIOMBs gravitational settling in tetrachloroethylene (density, 1.63
g/cm3) due to the density mismatch (Fig. 4a). In order to decrease the density, CIOMBs are dipped in the concentrated HCl (12
M). The iron oxide nanoparticles instead of carbon in the composite can react with
HCl, and the product can dissolve in the solution. Thus, the density of CIOMBs decreases
during the HCl etching. When the duration of etching is 1 h, the CIOMBs can be well
dispersed in tetrachloroethylene, whose density is about 1.7 g/cm3, which is a good density match. When the duration of etching is 12 h, the density
of CIOMBs is too light to be dispersed in tetrachloroethylene. Most of the CIOMBs
float on the surface of the suspension. From the TEM pictures, the decrease in iron
oxide content in the composite can be distinguished.

Figure 4.a CIOMBs with different etching time (from left to right 0, 1, 12 h) are dispersed
in tetrachloroethylene. TEM images of CIOMBs with 1 h (b) and 12 h etching

Exploiting their well dispersibility in tetrachloroethylene, CIOMBs are used as black
electrophoretic particles for electrophoretic displays. Electrophoretic dispersions
are prepared via dispersing of sample CIOMBs and titania (served as white electrophoretic
particles) in tetrachloroethylene using DISERBYK-161 as a dispersant. After above
dispersions are introduced into an electrophoretic cell, an electrophoretic display
is formed. Fig. 5 presents photographs of an electrophoretic display with dimensions of 75 μm (t) × 7 cm (w) × 3 cm (l) after applying 30 V.

Conclusions

In summary, using aqueous solutions containing ferrous chloride and glucose as precursors,
a simple and continuous strategy is developed to synthesize CIOMBs via USP. Due to
the presence of affluent functional groups in their surface, CIOMBs can be dispersed
in water, methanol, ethanol and tetrachloroethylene. By adjusting the concentrated
HCl etching duration, the density of CIOMBs can be tuned to match that of tetrachloroethylene.
The surface chemistry and density of as-prepared CIOMBs are appropriate for EPDs assembly.
An EPD is formed using CIOMBs as black electrophoretic particles in tetrachloroethylene.

Acknowledgments

This work was financially supported by the National Hi-Tech 863 Programme (2009AA03Z302),
National Science Foundation of China (60736001, 60907042), and Beijing Natural Science
Foundation (2093044).

Open Access

This article is distributed under the terms of the Creative Commons Attribution Noncommercial
License which permits any noncommercial use, distribution, and reproduction in any
medium, provided the original author(s) and source are credited.

References

Comiskey B, Albert JD, Hidekazu Y, Joseph J:

Nature. 1998, 394:253.

COI number [1:CAS:528:DyaK1cXkvVGqtrw%3D]; Bibcode number [1998Natur.394..253C]